This invention relates to a disk reproducing system such as a CD player, CD-ROM, DVD-ROM, and more particularly to a focus servo control circuit used in a DVD system.
A disk reproducing system used in an audio/visual device or computer peripheral device is constructed as shown by a system diagram of FIG. 1. A beam emitted from a laser pickup (PU) 12 is applied to a pit string on a disk 11 rotated by a disk motor 13 and a beam reflected therefrom is converted into an electrical signal (RF signal) by use of an optical element such as a photodiode to obtain pit data. The converted RF signal is amplified by an RF amplifier 15. The amplified RF signal is binary-coded into a digital signal and then supplied to a PLL (Phase Lock Loop) circuit in a data extraction circuit 26.
In the PLL circuit, a bit clock signal extracted with a data signal used as a reference is created. The data signal is processed together with the bit clock signal in a signal processing circuit 26. The signal processing circuit 26 is a circuit for receiving the data signal and effecting the processes of synchronization pattern detection, data demodulation, and error correction by use of a correction RAM 27 and video/audio data or computer data can be output to the exterior only after it is subjected to the above processes.
Referring to FIG. 1, a system associated with the focus servo control circuit is explained. A signal read out from the disk 11 by use of the pickup 12 is input to the RF amplifier (RF AMP) 15 and a tracking error signal TE, focus error signal FE and RF signal which is an information signal are derived from the RF amplifier 15.
The tracking error signal TE is input to a tracking servo control circuit 16, the gain and phase thereof are compensated for and the seek control or the like is effected. An output signal of the tracking servo control circuit 16 is input to an actuator driver 19 to drive the tracking actuator of the pickup 12. Further, an output signal of an output equalizer of the tracking servo control circuit 16 is also supplied to a feed motor control circuit 17 to drive a feed motor 14 via a motor driver 18.
The focus error signal FE is input to a focus servo equalizer 21 via a servo ON/OFF switch 20. An output signal of the focus servo equalizer 21 is added to a generation signal of a lens drive signal generating circuit 23 and the result of addition is supplied to a focus actuator driver 22. Then, it is supplied to drive the focus actuator of the pickup 12.
The RF signal is supplied to a data extraction circuit and CD/DVD data signal processing circuit 26, binary-coded in the data extraction circuit, then demodulated after a bit clock and synchronizing signal are extracted, and subjected to the correction process by use of the correction RAM 27. The synchronizing signal is supplied to a disk motor control circuit 24 to control a disk motor 13 via a disk motor driver 25.
Data corrected in the data signal processing circuit 26 is supplied to an MPEG video/audio decoder processing circuit 28 in the case of DVD movie and a video signal or audio signal is output from the circuit. In the case of DVD-ROM, output data from the data signal processing circuit 26 is supplied to a data buffer circuit 28 and data is output therefrom as digital data to a host personal computer or the like. A system controller 29 controls the control timings of the various control circuits and the operation of the whole set.
FIG. 2 is a timing diagram showing the movement of a beam spot (lens) and the movement of various signals at the time of movement between layers (which is hereinafter referred to as layer jump). In FIG. 2, D indicates a distance between the first and second layers and is approx. 55 .mu.m, for example, in the case of DVD system. (A) indicates a case wherein the direction of the surface deviation of the disk is the same as the moving direction (upward direction in this example) of the beam spot. The speed of the beam spot in the layer direction is relatively low with respect to the disk surface.
(B) and (C) indicate cases wherein the direction of the surface deviation of the disk is opposite to the moving direction (downward direction in this example) of the beam spot. The speed of the beam spot in the layer direction is relatively high with respect to the disk surface.
Thus, since the moving speed condition of the beam spot is changed depending on the position even on the same disk, the layer jump may fail sometimes when the focus servo is turned ON again on the layer of the destination. In FIG. 2, (A) and (B) show the successful case and (C) shows the failure case. In the case of (C), the beam spot is separated from the layer of the destination.
However, the layer jump operation of the focus servo is an extremely important operation in the DVD system, and since only two layers are provided, it cannot be settled on the other layer in the case of failure of the layer jump unlike the track jump of the tracking servo. As a result, the result of failure becomes serious. Further, in the case of failure, since the central control unit is required to restore the servo to the initial state and effect the operation starting from the focus search, it takes a long time to recover the operating state. Whether the phenomenon occurs or not is determined by the surface deviation distance and rotation speed.
From the above points, it is desired to enhance the accuracy of the layer jump operation.